In the field of drug delivery science, there is a recent trend in developing nanoparticle (NP) formulations that can simultaneously deliver two or more drugs with different therapeutic targets (i.e. co-delivery). Although such an approach in pharmaceutical drug development is scientifically attractive, the concept remains to be technically challenging. First, the loaded drugs often have distinct physicochemical properties (e.g. hydrophobic vs. hydrophilic); therefore, it is difficult, if not impossible, to achieve the same loading efficiency within a polymeric carrier material. Second, to achieve effective combination therapy, it is desirable to tune the release profiles of the co-loaded drugs, so that one drug may release faster and the other slower. However, within the same nanostructure, this cannot be easily achieved. Third, potential chemical interaction should be considered when two drugs are co-existing within the same nanoparticle confinement.